Summary of Techniques
Problems attributable to electromagnetic inference (EMI) have increased with the recent functional progress and spread of electric and electronic devices. Electromagnetic noise is classified roughly into conducted noise and radiated noise. Methods of preventing problems due to conducted noise filter conducted noise with a noise filter. Methods of preventing problems due to radiated noise use a metal case to shield a space electromagnetically, place a metal sheet between wiring boards or coat the wires of cables with a metal foil. Although effective in electromagnetically shielding circuits and power blocks, these methods are unsuitable for shielding electromagnetic radiation generated by the screens of displays, such as CRTs and PDPs, because these methods use opaque means.
The PDP is an assembly of a glass panel provided with data electrodes and a fluorescent layer, and a glass panel provided with transparent electrodes. The PDP generates a large amount of electromagnetic radiation, a large amount of near-infrared radiation and a large amount of heat when operated. Usually, a front panel is disposed in front of the PDP to shield electromagnetic radiation. The front panel must have a shielding function of 30 dB or above in the range of 30 MHz to 1 GHz to shield electromagnetic radiation emitted from the screen of the display. Infrared radiation of wavelengths in the range of 800 to 1,200 nm radiated from the screen of the display must be shielded because infrared radiation makes other devices, such as VTRs, malfunction. The electromagnetic shielding metal mesh (lines) of the electromagnetic shielding sheet must be hardly visible to facilitate the observation of images displayed on the screen of the display, and the electromagnetic shielding sheet must have proper transparency (visible light transmitting property, visible light transmittance) to make images displayed by the display satisfactorily visible. However, there have not been electromagnetic shielding sheets capable of practically providing both the desired electromagnetic shielding property and the desired transparency, not to mention the desired electromagnetic shielding property, the desired transparency and infrared shielding property.
Prior Art
The front panel must be provided with a conductive member for electromagnetic shielding and must have proper transparency (visible light transmittance). That is, the front panel must have both electromagnetic shielding ability and transparency. Conductive members are classified roughly into those including a transparent thin film of a conductive metal or a metal oxide formed on a transparent plate, or those having a mesh structure having fine lines of an opaque conductive material.
The electromagnetic shielding sheet provided with a conductive, transparent thin film of a metal or a metal oxide does not have high electromagnetic shielding ability, but has a high transmittance. Conductive thin film-forming methods disclosed in JP 1-278800 A and JP 5-323101 A form a conductive thin film of a conductive metal or a metal oxide on a transparent base by vapor deposition. While the deposited conductive thin film of the metal or the metal oxide is formed not excessively thick to meet the required transparency, such as a thickness in the range of several hundreds to 2000 Å, the conductive thin film has an excessively high surface electrical resistance and is unsatisfactory in electromagnetic shielding ability.
The electromagnetic shielding sheet provided with a mesh structure having fine lines has high electromagnetic shielding ability, but is subject to limitations on reducing the fineness of the fine lines to increase transmittance. Such fine lines are difficult to form. Electromagnetic shielding sheets formed by embedded fibers having high conductivity in a transparent base are disclosed in JP 5-327274 A and JP 5-269912 A. Though those electromagnetic shielding sheets have satisfactory electromagnetic shielding ability, the regularly arranged conductive fibers for electromagnetic shielding have excessively large diameters not less than 35 μm. Such thick conductive fibers are visible and deteriorate the visibility of displayed images.
Electromagnetic shielding sheets disclosed in JP 62-57297 A and JP 2-52499 A are formed by printing lines of a conductive resin containing metal powder or the like directly on a transparent base plate. The lines of the conductive resin have wide widths on the order of 100 μm due to limited printing accuracy. Such wide lines deteriorate the visibility of displayed images.
To improve the visibility of displayed images, reflection of light from the surface of the conductive film of the electromagnetic shielding sheet must be reduced, and a blackened layer needs to be formed over the surface facing the viewer of the conductive film. An electromagnetic shielding sheet disclosed in JP 5-283889 A has a structure: (base)/(transparent anchor layer)/(electromagnetic shielding layer). The electromagnetic shielding layer has a meshed pattern and is formed by an electroless plating process. The transparent anchor layer underlying the electromagnetic shielding layer is blackened. A reflection reducing method disclosed in JP 61-15480 A forms a copper oxide film on a metal mesh included in an electromagnetic shielding sheet to reduce the reflection of external light. A method disclosed in JP 09-293989 A uses a meshed black resist film used for forming a metal mesh included in an electromagnetic shielding sheet by photolithography as a meshed black frame (lines). An electromagnetic shielding structure disclosed in JP 10-335885 A is formed by laminating a plastic film provided with a copper foil patterned in a geometrical pattern by photolithography to a plastic sheet. However, the density of the black metal meshed frames (lines) of those electromagnetic sheets formed by these prior art methods is low, and the visibility of images displayed by displays provided with these electromagnetic shielding sheets is unsatisfactory.